The present invention relates to producing large, controllable, vibratory forces to compensate for sensed noise or vibrations, and more particularly to an active vibration control (AVC) system for an aircraft.
The dominant source of vibration in a helicopter in forward flight is that generated by the main rotor system rotating at the blade passing frequency. Forces and moments are transmitted usually through the transmission via fuselage attachments, to produce vibration in the fuselage.
One conventional approach to reducing such vibration involves replacing a rigid gearbox mounting strut with a compliant strut and parallel hydraulic actuator. A control computer commands the actuators such that the gearbox is selectively vibrated to produce inertial forces which minimize fuselage vibrations. Although effective, this approach is inadequate in a vehicle having a gearbox which is directly attached to the airframe i.e., without struts.
Another conventional approach utilizes counter-rotating eccentric masses that rotate at the frequency of the primary aircraft vibration and generate a fixed magnitude vibration force. A second pair of eccentric masses is phased relative to the first pair to yield any force magnitude from zero to maximum force. This system, although effective for direct gearbox mounting, requires a parasitic mass of considerable magnitude which results in an unacceptable weight penalty. Moreover, this approach does not provide an acceptable reduction in size as the diameter of the circular shaped device is difficult to fit in the confined spaces available in an aircraft.
Accordingly, it is desirable to provide an active vibration control system which generates relatively large controllable vibratory forces with a lower weight and smaller size than conventional systems.